1. Field of the Invention
The present invention relates to a current detection circuit and a current detection method using a multi-source MOS transistor.
2. Description of Related Art
In recent years, electronic devices mounted in a vehicle have become diversified, and the number of such devices mounted in the vehicle has been increasing. When one of such electronic devices such as a headlamp or a brake lamp becomes unable to continue the normal operation owing to a broken wire or the like, it can lead to an accident. Therefore, mounting a detection device to detect a broken wire is indispensable, and it has been made obligatory by law. Furthermore, LEDs (Light Emitting Diodes), which have smaller current consumption than that of halogen lamps, have been taking the place of the halogen lamps for each of various lamps and the like. To detect a broken wire regardless of whether the connected load is a halogen lamp or a LED, it is important to accurately detect the load current flowing from the power device.
Japanese Unexamined Patent Application Publication No. 2003-28901 discloses a detection method of load current using a multi-source MOS transistor. FIG. 5 is a circuit to detect load current using the multi-source MOS transistor, and it is composed of the multi-source MOS transistor having a load MOS transistor and a detection MOS transistor, and a current detection circuit including an operational amplifier. By establishing a predetermined ratio between the on-resistances of the load MOS transistor and the detection MOS transistor and setting the on-resistance of the detection MOS transistor larger than that of the load MOS transistor, and using the fact that the voltage between the drain and the source of the detection MOS transistor is made substantially equal to that of the load MOS transistor by the operational amplifier, the current flowing through the current detection circuit is establish such that it has a predetermined ratio to the load current flowing through the load MOS transistor and the amount of it becomes very small. Therefore, the load current can be detected without having any adverse effect on the current consumption.
Since an operational amplifier is used as the current detection circuit in the load current detection circuit using the multi-source MOS transistor, it poses a problem that the accuracy is affected by the offset of the operational amplifier. Japanese Unexamined Patent Application Publication No. 2003-28901, as shown in FIG. 6, adds a MOS resistance changing means to the load current detection circuit using the. multi-source MOS transistor shown in FIG. 5 in an attempt to solve the problem relating to the offset of the operational amplifier. The MOS resistance changing means controls the on-resistances of the load MOS transistor and the detection MOS transistor such that the voltage drop by the load current at the load MOS transistor and the detection MOS transistor is maintained constant at all times. The offset of an operational amplifier has the maximum effect when the voltage drop at the load MOS transistor and the detection MOS transistor is small. Therefore, Japanese Unexamined Patent Application Publication No. 2003-28901 reduces the effect of the offset of the operational amplifier by controlling the gate bias voltage to the load MOS transistor and the detection MOS transistor, i.e., by controlling the amount of the voltage drop at the load MOS transistor and the detection MOS transistor. However, this method cannot completely cancel the offset.
Japanese Unexamined Patent Application Publication No. 2003-28901 further discloses a current detection method, like one shown in FIG. 7, in order to cancel the offset of the operational amplifier. Assume that the sense ratio of the detection MOS transistor and the load MOS transistor is 1: n, and the offset voltage of the operational amplifier is Voff. Then, the current flowing through the current detection circuit is measured twice, once for when the on-resistance of the load MOS transistor is set to Ron, and once for when it is set to 0.5 Ron. Assuming the load current is I, the current which is measured in twice is expressed in the following equations.Iout1=(I×Ron+Voff)/(n×Ron)Iout2=(I×0.5Ron+Voff)/(n×0.5Ron)The load current I is expressed in the following equation from these two equations.I=(2Iout1−Iout2)/n That is, Japanese Unexamined Patent Application Publication No. 2003-28901 states that the load current for which the offset Voff is cancelled can be obtained from the calculation of two current measurements.
It also states that the load current for which the offset of the operational amplifier is cancelled can be detected by the current detection circuit shown in FIG. 7 of Japanese Unexamined Patent Application Publication No. 2003-28901. However, as can be seen from above-described two equations, it requires that the load current I should not be changed between the two current measurements for Iout1 and Iout2. If the load current I is changed between at the measurement for Iout1 and at the measurement for Iout2, it becomes very difficult to obtain the load current I from Iout1 and Iout2. In the case of Japanese Unexamined Patent Application Publication No. 2003-28901, since the measurements are carried out with different on-resistances of the multi-source MOS transistor (Ron and 0.5 Ron in the embodiment), the load current I is not necessarily the same between the measurements for Iout1 and Iout2 unless other conditions such as the power supply and the load are changed accordingly.
Furthermore, there is a possibility that the load current may be changed between the two current measurements for Iout1 and Iout2 simply owing to a broken wire in the device connected as the load or a similar condition. The measurement of the load current is unreliable in such cases. In such cases, it is necessary to take measures to determine the normality/abnormality of the measurement result, and to re-measuring Iout1 and Iout2 when the original measurement is determined to be abnormal, or similar measures.